where can I find this in the python script?
To find the source that displays a property,  RMB on it and choose edit source. If blender has a visible text editor it will show the script with the cursor on the relevant line, if no text editor is visible you will get a notice at the top of the window to see the script in the text editor.



Note that the UI scripts you will get only display existing values, if you alter the UI script to always turn the value on you will never be able to turn it off.

I would suggest making an addon that creates new objects and new materials with settings the way you want. This answer shows an example of injecting a button into an existing panel.